The trend to thinner and thinner mobile phones, as well as to increasing resolutions, leads to lens modules with particular designs, a lens module being constituted by an assembly of one to several lenses and diaphragms into a lens holder. Also, for modules aiming at large volumes markets a particular attention must be laid on the manufacturability, because a production of several tens of thousands of lens modules per day can be envisaged only when the manufacturing yield is close to 100%.
Depending on the customer's specifications, the constraints that have the major influence on the design are:
Effective Focal Length
The Effective Focal Length determines the overall dimension of the module. The Effective Focal Length will hereafter be referred to as EFL.
Combined with the dimension of the image that will be formed on the sensor that is used with a module, the EFL determines also the Field of View, referred to as FOV, and combined with the diameter of the Aperture diaphragm of the module, if present (or eventually the diameter of the lenses, if no diaphragm is present), determines the Aperture Number of the module, referred to as F#.
When a circular image of diameter D is formed in the focal plane of the module, the FOV is defined by the relation:FOV=2·Arctan(D/[2·EFL])
where Arctan is the inverse function of the tangent of an angle.
The F# has a major influence on four important parameters: the light reaching the sensor, which varies as the inverse of the square of the F#, the Depth of Field (DoF), the Hyperfocal distance (HyF) which is the minimum distance at which an object gives a neat image on the sensor, and the depth of focus (dof) which is the tolerance on the position of the sensor with respect to the lens module.
Resolution
The resolution is measured by the Modulation Transfer Function (MTF) at a given spatial frequency. The resolution characterizes the contrast between the black area and the white area in an image composed of a series of alternatively black and white stripes of equal width, the width of a pair of stripes being the inverse of the spatial frequency.
Aberrations
The aberrations are both geometric and chromatic.
The geometric aberrations include the geometric distortion, the astigmatism, and the EFL differences between various areas of the image. They depend on the curvature on axis of the lenses, and on the asphericity coefficients that define, at a given distance of the axis, the distance between the surface of a sphere having the same radius on axis and the surface of the lens.
The chromatic aberrations include the “colored fringes” (edges of an object are surrounded by parallel edges of various colors) and the “colored area” (a white image presents for example pink corners).
Targeted Costs
The targeted cost of the module depends primarily on the number of lenses that compose the module. The present invention allows reducing the aberration by defining constraints on the curvature of the lenses, rather than by adding more lenses.
Maximum Temperature
The maximum temperature range that the module can withstand during a given period of time without damage is an extremely important parameter, as the trend in the manufacturing of mobile phones is to solder all the components, including the optical module, in one single operation called “reflow” which supposes that the components withstand a temperature of 260 degree Celsius during 30 seconds, and 230 degree Celsius during 60 seconds.
Up to recently, the lenses of the optical components of high quality have been done using lenses with aspherical surfaces, made of thermoplastic materials like Polycarbonate, or Cyclo Olefin Polymer, which do not withstand such temperatures. The manufacturing of the mobile phones using such components has to be done in two steps: first soldering a socket, then inserting the module into the socket at ambient temperature, which increases the number of operations and the number of components in inventories, reduces the reliability and leads to increased costs.
With so many constraints, one can understand that a lens module is designed for a particular set of specifications. However, as very often for a given sensor, the phone makers develop several models with slightly different characteristics (slight variations on EFL, FOV, and MTF specifications). It is possible to design a module with some versatility by giving a range of variation to the various design parameters.
Many two-lens modules have been described.
A module is described in JP 2004-226595, which is composed of two positive meniscus lenses. The main characteristic of this module is to be made with resin lenses. Optical lenses made out of resin have not been widely accepted in the industry, due to reasons that are intrinsic to the material properties (limited range of refractive index, Abbe number too low, and control of the dimensions during the process).
Others two-lens modules are described in U.S. Pat. No. 6,011,660 and U.S. Pat. No. 5,739,965. These patents share the feature of having a first lens with a convex second surface.
Two others US patent, U.S. Pat. No. 6,842,295 and U.S. Pat. No. 5,801,890, describe two-lens modules sharing the feature of having a second lens with a concave image side surface.
Two Japanese patents, JP 2003-063786 and JP 3588518, describe a two-lens structure, the first lens having a negative power.
A Japanese patent, JP 2003-041258, describes a two-lens module, the second lens having the object side surface convex, and the image side surface concave near the axis and convex near the periphery.
Two US patent, U.S. Pat. No. 6,650,485 and U.S. Pat. No. 5,666,234, describe a two-lens module, the first lens being bi convex.
There are also many others publications describing two-lens modules, using a combination of convex and concave surfaces different from the present invention; for example: U.S. Pat. No. 5,835,288, U.S. Pat. No. 5,801,890, U.S. Pat. No. 6,011,660, U.S. Pat. No. 6,104,553, U.S. Pat. No. 6,842,295, U.S. Pat. No. 6,873,474, U.S. Pat. No. 7,035,018, U.S. Pat. No. 6,876,500, U.S. Pat. No. 7,061,696, U.S. Pat. No. 7,031,080.
Japanese application JP 3027863 and U.S. Pat. No. 5,067,803 describe a module composed of two positive meniscus lenses, with a restrictive condition on the ratio between the focal length fF of the first lens and the focal length fR of the second lens, along a ratio such that: 0.85<fF/fR<1.15, which differs from the present invention.
There exist a number of publications that describe modules composed of two lenses, with restrictive conditions on the focal lengths of the lenses, or on the asphericity coefficients of the surfaces, which are tailored for particular applications and that differ from the present invention; for example: U.S. Pat. No. 5,600,493, U.S. Pat. No. 5,739,965, U.S. Pat. No. 6,335,835, U.S. Pat. No. 6,628,463, U.S. Pat. No. 6,577,456, U.S. Pat. No. 6,650,485, U.S. Pat. No. 6,882,483.